At all times, PostgreSQL
maintains a write ahead log (WAL) in the
pg_xlog/ subdirectory of the cluster's
data directory. The log describes every change made to the
database's data files. This log exists primarily for crash-safety
purposes: if the system crashes, the database can be restored to
consistency by "replaying" the log
entries made since the last checkpoint. However, the existence of
the log makes it possible to use a third strategy for backing up
databases: we can combine a file-system-level backup with backup
of the WAL files. If recovery is needed, we restore the backup
and then replay from the backed-up WAL files to bring the backup
up to current time. This approach is more complex to administer
than either of the previous approaches, but it has some
significant benefits:

We do not need a perfectly consistent backup as the
starting point. Any internal inconsistency in the backup will
be corrected by log replay (this is not significantly
different from what happens during crash recovery). So we
don't need file system snapshot capability, just tar or a similar archiving tool.

Since we can string together an indefinitely long sequence
of WAL files for replay, continuous backup can be achieved
simply by continuing to archive the WAL files. This is
particularly valuable for large databases, where it may not
be convenient to take a full backup frequently.

There is nothing that says we have to replay the WAL
entries all the way to the end. We could stop the replay at
any point and have a consistent snapshot of the database as
it was at that time. Thus, this technique supports point-in-time recovery: it is possible to
restore the database to its state at any time since your base
backup was taken.

If we continuously feed the series of WAL files to another
machine that has been loaded with the same base backup file,
we have a "hot standby" system: at
any point we can bring up the second machine and it will have
a nearly-current copy of the database.

As with the plain file-system-backup technique, this method
can only support restoration of an entire database cluster, not a
subset. Also, it requires a lot of archival storage: the base
backup may be bulky, and a busy system will generate many
megabytes of WAL traffic that have to be archived. Still, it is
the preferred backup technique in many situations where high
reliability is needed.

To recover successfully using an on-line backup, you need a
continuous sequence of archived WAL files that extends back at
least as far as the start time of your backup. So to get started,
you should set up and test your procedure for archiving WAL files
before you take your
first base backup. Accordingly, we first discuss the mechanics of
archiving WAL files.

In an abstract sense, a running PostgreSQL system produces an indefinitely
long sequence of WAL records. The system physically divides
this sequence into WAL segment files,
which are normally 16MB apiece (although the size can be
altered when building PostgreSQL). The segment files are given
numeric names that reflect their position in the abstract WAL
sequence. When not using WAL archiving, the system normally
creates just a few segment files and then "recycles" them by renaming no-longer-needed
segment files to higher segment numbers. It's assumed that a
segment file whose contents precede the checkpoint-before-last
is no longer of interest and can be recycled.

When archiving WAL data, we want to capture the contents of
each segment file once it is filled, and save that data
somewhere before the segment file is recycled for reuse.
Depending on the application and the available hardware, there
could be many different ways of "saving the
data somewhere": we could copy the segment files to an
NFS-mounted directory on another machine, write them onto a
tape drive (ensuring that you have a way of restoring the file
with its original file name), or batch them together and burn
them onto CDs, or something else entirely. To provide the
database administrator with as much flexibility as possible,
PostgreSQL tries not to make
any assumptions about how the archiving will be done. Instead,
PostgreSQL lets the
administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command
could be as simple as a cp, or
it could invoke a complex shell script — it's all up to
you.

The shell command to use is specified by the archive_command
configuration parameter, which in practice will always be
placed in the postgresql.conf file.
In this string, any %p is replaced by
the path name of the file to archive, while any %f is replaced by the file name only. (The path
name is relative to the working directory of the server, i.e.,
the cluster's data directory.) Write %% if you need to embed an actual % character in the command. The simplest useful
command is something like

archive_command = 'cp -i %p /mnt/server/archivedir/%f </dev/null'

which will copy archivable WAL segments to the directory
/mnt/server/archivedir. (This is an
example, not a recommendation, and may not work on all
platforms.)

The archive command will be executed under the ownership of
the same user that the PostgreSQL server is running as. Since the
series of WAL files being archived contains effectively
everything in your database, you will want to be sure that the
archived data is protected from prying eyes; for example,
archive into a directory that does not have group or world read
access.

It is important that the archive command return zero exit
status if and only if it succeeded. Upon getting a zero result,
PostgreSQL will assume that
the WAL segment file has been successfully archived, and will
remove or recycle it. However, a nonzero status tells
PostgreSQL that the file was
not archived; it will try again periodically until it
succeeds.

The archive command should generally be designed to refuse
to overwrite any pre-existing archive file. This is an
important safety feature to preserve the integrity of your
archive in case of administrator error (such as sending the
output of two different servers to the same archive directory).
It is advisable to test your proposed archive command to ensure
that it indeed does not overwrite an existing file,
and that it returns nonzero
status in this case. We have found that cp -i does this correctly on some platforms but
not others. If the chosen command does not itself handle this
case correctly, you should add a command to test for
pre-existence of the archive file. For example, something
like

archive_command = 'test ! -f .../%f && cp %p .../%f'

works correctly on most Unix variants.

While designing your archiving setup, consider what will
happen if the archive command fails repeatedly because some
aspect requires operator intervention or the archive runs out
of space. For example, this could occur if you write to tape
without an autochanger; when the tape fills, nothing further
can be archived until the tape is swapped. You should ensure
that any error condition or request to a human operator is
reported appropriately so that the situation can be resolved
relatively quickly. The pg_xlog/
directory will continue to fill with WAL segment files until
the situation is resolved.

The speed of the archiving command is not important, so long
as it can keep up with the average rate at which your server
generates WAL data. Normal operation continues even if the
archiving process falls a little behind. If archiving falls
significantly behind, this will increase the amount of data
that would be lost in the event of a disaster. It will also
mean that the pg_xlog/ directory will
contain large numbers of not-yet-archived segment files, which
could eventually exceed available disk space. You are advised
to monitor the archiving process to ensure that it is working
as you intend.

If you are concerned about being able to recover right up to
the current instant, you may want to take additional steps to
ensure that the current, partially-filled WAL segment is also
copied someplace. This is particularly important if your server
generates only little WAL traffic (or has slack periods where
it does so), since it could take a long time before a WAL
segment file is completely filled and ready to archive. One
possible way to handle this is to set up a cron job that periodically (once a minute,
perhaps) identifies the current WAL segment file and saves it
someplace safe. Then the combination of the archived WAL
segments and the saved current segment will be enough to ensure
you can always restore to within a minute of current time. This
behavior is not presently built into PostgreSQL because we did not want to
complicate the definition of the archive_command
by requiring it to keep track of successively archived, but
different, copies of the same WAL file. The archive_command
is only invoked on completed WAL segments. Except in the case
of retrying a failure, it will be called only once for any
given file name.

In writing your archive command, you should assume that the
file names to be archived may be up to 64 characters long and
may contain any combination of ASCII letters, digits, and dots.
It is not necessary to remember the original full path
(%p) but it is necessary to remember
the file name (%f).

Note that although WAL archiving will allow you to restore
any modifications made to the data in your PostgreSQL database it will not restore
changes made to configuration files (that is, postgresql.conf, pg_hba.conf and pg_ident.conf), since those are edited manually
rather than through SQL operations. You may wish to keep the
configuration files in a location that will be backed up by
your regular file system backup procedures. See Section 17.2 for how
to relocate the configuration files.

where label is any string you
want to use to uniquely identify this backup operation.
(One good practice is to use the full path where you intend
to put the backup dump file.) pg_start_backup creates a backup label file, called backup_label, in the cluster directory with
information about your backup.

It does not matter which database within the cluster you
connect to to issue this command. You can ignore the result
returned by the function; but if it reports an error, deal
with that before proceeding.

Perform the backup, using any convenient
file-system-backup tool such as tar or cpio. It is neither necessary nor
desirable to stop normal operation of the database while
you do this.

Again connect to the database as a superuser, and issue
the command

SELECT pg_stop_backup();

This should return successfully.

Once the WAL segment files used during the backup are
archived as part of normal database activity, you are
done.

Some backup tools that you might wish to use emit warnings
or errors if the files they are trying to copy change while the
copy proceeds. This situation is normal, and not an error, when
taking a base backup of an active database; so you need to
ensure that you can distinguish complaints of this sort from
real errors. For example, some versions of rsync return a separate exit code for
"vanished source files", and you can
write a driver script to accept this exit code as a non-error
case. Also, some versions of GNU tar consider it an error if a file is
changed while tar is copying
it. There does not seem to be any very convenient way to
distinguish this error from other types of errors, other than
manual inspection of tar's
messages. GNU tar is therefore
not the best tool for making base backups.

It is not necessary to be very concerned about the amount of
time elapsed between pg_start_backup and the start of the actual
backup, nor between the end of the backup and pg_stop_backup; a few minutes' delay won't
hurt anything. You must however be quite sure that these
operations are carried out in sequence and do not overlap.

Be certain that your backup dump includes all of the files
underneath the database cluster directory (e.g., /usr/local/pgsql/data). If you are using
tablespaces that do not reside underneath this directory, be
careful to include them as well (and be sure that your backup
dump archives symbolic links as links, otherwise the restore
will mess up your tablespaces).

You may, however, omit from the backup dump the files within
the pg_xlog/ subdirectory of the
cluster directory. This slight complication is worthwhile
because it reduces the risk of mistakes when restoring. This is
easy to arrange if pg_xlog/ is a
symbolic link pointing to someplace outside the cluster
directory, which is a common setup anyway for performance
reasons.

To make use of this backup, you will need to keep around all
the WAL segment files generated during and after the file
system backup. To aid you in doing this, the pg_stop_backup function creates a backup history file that is immediately stored
into the WAL archive area. This file is named after the first
WAL segment file that you need to have to make use of the
backup. For example, if the starting WAL file is 0000000100001234000055CD the backup history file
will be named something like 0000000100001234000055CD.007C9330.backup. (The
second number in the file name stands for an exact position
within the WAL file, and can ordinarily be ignored.) Once you
have safely archived the file system backup and the WAL segment
files used during the backup (as specified in the backup
history file), all archived WAL segments with names numerically
less are no longer needed to recover the file system backup and
may be deleted. However, you should consider keeping several
backup sets to be absolutely certain that you can recover your
data. Keep in mind that only completed WAL segment files are
archived, so there will be delay between running pg_stop_backup and the archiving of all WAL
segment files needed to make the file system backup
consistent.

The backup history file is just a small text file. It
contains the label string you gave to pg_start_backup, as well as the starting and
ending times and WAL segments of the backup. If you used the
label to identify where the associated dump file is kept, then
the archived history file is enough to tell you which dump file
to restore, should you need to do so.

Since you have to keep around all the archived WAL files
back to your last base backup, the interval between base
backups should usually be chosen based on how much storage you
want to expend on archived WAL files. You should also consider
how long you are prepared to spend recovering, if recovery
should be necessary — the system will have to replay all those
WAL segments, and that could take awhile if it has been a long
time since the last base backup.

It's also worth noting that the pg_start_backup function makes a file named
backup_label in the database cluster
directory, which is then removed again by pg_stop_backup. This file will of course be
archived as a part of your backup dump file. The backup label
file includes the label string you gave to pg_start_backup, as well as the time at which
pg_start_backup was run, and the
name of the starting WAL file. In case of confusion it will
therefore be possible to look inside a backup dump file and
determine exactly which backup session the dump file came
from.

It is also possible to make a backup dump while the
postmaster is stopped. In this case, you obviously cannot use
pg_start_backup or pg_stop_backup, and you will therefore be
left to your own devices to keep track of which backup dump is
which and how far back the associated WAL files go. It is
generally better to follow the on-line backup procedure
above.

Okay, the worst has happened and you need to recover from
your backup. Here is the procedure:

Stop the postmaster, if it's running.

If you have the space to do so, copy the whole cluster
data directory and any tablespaces to a temporary location
in case you need them later. Note that this precaution will
require that you have enough free space on your system to
hold two copies of your existing database. If you do not
have enough space, you need at the least to copy the
contents of the pg_xlog
subdirectory of the cluster data directory, as it may
contain logs which were not archived before the system went
down.

Clean out all existing files and subdirectories under
the cluster data directory and under the root directories
of any tablespaces you are using.

Restore the database files from your backup dump. Be
careful that they are restored with the right ownership
(the database system user, not root!) and with the right
permissions. If you are using tablespaces, you may want to
verify that the symbolic links in pg_tblspc/ were correctly restored.

Remove any files present in pg_xlog/; these came from the backup dump
and are therefore probably obsolete rather than current. If
you didn't archive pg_xlog/ at
all, then re-create it, and be sure to re-create the
subdirectory pg_xlog/archive_status/ as well.

If you had unarchived WAL segment files that you saved
in step 2, copy them into pg_xlog/. (It is best to copy them, not
move them, so that you still have the unmodified files if a
problem occurs and you have to start over.)

Create a recovery command file recovery.conf in the cluster data directory
(see Recovery
Settings). You may also want to temporarily modify
pg_hba.conf to prevent ordinary
users from connecting until you are sure the recovery has
worked.

Start the postmaster. The postmaster will go into
recovery mode and proceed to read through the archived WAL
files it needs. Upon completion of the recovery process,
the postmaster will rename recovery.conf to recovery.done (to prevent accidentally
re-entering recovery mode in case of a crash later) and
then commence normal database operations.

Inspect the contents of the database to ensure you have
recovered to where you want to be. If not, return to step
1. If all is well, let in your users by restoring
pg_hba.conf to normal.

The key part of all this is to set up a recovery command
file that describes how you want to recover and how far the
recovery should run. You can use recovery.conf.sample (normally installed in the
installation share/ directory) as a
prototype. The one thing that you absolutely must specify in
recovery.conf is the restore_command, which tells PostgreSQL how to get back archived WAL
file segments. Like the archive_command, this is a shell command string.
It may contain %f, which is replaced
by the name of the desired log file, and %p, which is replaced by the path name to copy
the log file to. (The path name is relative to the working
directory of the server, i.e., the cluster's data directory.)
Write %% if you need to embed an
actual % character in the command. The
simplest useful command is something like

restore_command = 'cp /mnt/server/archivedir/%f %p'

which will copy previously archived WAL segments from the
directory /mnt/server/archivedir. You
could of course use something much more complicated, perhaps
even a shell script that requests the operator to mount an
appropriate tape.

It is important that the command return nonzero exit status
on failure. The command will be asked for log files that
are not present in the archive; it must return nonzero when so
asked. This is not an error condition. Be aware also that the
base name of the %p path will be
different from %f; do not expect them
to be interchangeable.

WAL segments that cannot be found in the archive will be
sought in pg_xlog/; this allows use
of recent un-archived segments. However segments that are
available from the archive will be used in preference to files
in pg_xlog/. The system will not
overwrite the existing contents of pg_xlog/ when retrieving archived files.

Normally, recovery will proceed through all available WAL
segments, thereby restoring the database to the current point
in time (or as close as we can get given the available WAL
segments). But if you want to recover to some previous point in
time (say, right before the junior DBA dropped your main
transaction table), just specify the required stopping point in
recovery.conf. You can specify the
stop point, known as the "recovery
target", either by date/time or by completion of a
specific transaction ID. As of this writing only the date/time
option is very usable, since there are no tools to help you
identify with any accuracy which transaction ID to use.

Note: The stop point must be after the ending
time of the base backup (the time of pg_stop_backup). You cannot use a base
backup to recover to a time when that backup was still
going on. (To recover to such a time, you must go back to
your previous base backup and roll forward from there.)

These settings can only be made in the recovery.conf file, and apply only for the
duration of the recovery. They must be reset for any
subsequent recovery you wish to perform. They cannot be
changed once recovery has begun.

restore_command (string)

The shell command to execute to retrieve an archived
segment of the WAL file series. This parameter is
required. Any %f in the string
is replaced by the name of the file to retrieve from
the archive, and any %p is
replaced by the path name to copy it to on the server.
(The path name is relative to the working directory of
the server, i.e., the cluster's data directory.) Write
%% to embed an actual
% character in the
command.

It is important for the command to return a zero
exit status if and only if it succeeds. The command
will be asked
for file names that are not present in the archive; it
must return nonzero when so asked. Examples:

This parameter specifies the time stamp up to which
recovery will proceed. At most one of recovery_target_time and recovery_target_xid
can be specified. The default is to recover to the end
of the WAL log. The precise stopping point is also
influenced by recovery_target_inclusive.

recovery_target_xid (string)

This parameter specifies the transaction ID up to
which recovery will proceed. Keep in mind that while
transaction IDs are assigned sequentially at
transaction start, transactions can complete in a
different numeric order. The transactions that will be
recovered are those that committed before (and
optionally including) the specified one. At most one of
recovery_target_xid and
recovery_target_time
can be specified. The default is to recover to the end
of the WAL log. The precise stopping point is also
influenced by recovery_target_inclusive.

recovery_target_inclusive (boolean)

Specifies whether we stop just after the specified
recovery target (true), or
just before the recovery target (false). Applies to both recovery_target_time
and recovery_target_xid,
whichever one is specified for this recovery. This
indicates whether transactions having exactly the
target commit time or ID, respectively, will be
included in the recovery. Default is true.

recovery_target_timeline (string)

Specifies recovering into a particular timeline. The
default is to recover along the same timeline that was
current when the base backup was taken. You would only
need to set this parameter in complex re-recovery
situations, where you need to return to a state that
itself was reached after a point-in-time recovery. See
Section
23.3.4 for discussion.

The ability to restore the database to a previous point in
time creates some complexities that are akin to science-fiction
stories about time travel and parallel universes. In the
original history of the database, perhaps you dropped a
critical table at 5:15PM on Tuesday evening. Unfazed, you get
out your backup, restore to the point-in-time 5:14PM Tuesday
evening, and are up and running. In this history of the database
universe, you never dropped the table at all. But suppose you
later realize this wasn't such a great idea after all, and
would like to return to some later point in the original
history. You won't be able to if, while your database was
up-and-running, it overwrote some of the sequence of WAL
segment files that led up to the time you now wish you could
get back to. So you really want to distinguish the series of
WAL records generated after you've done a point-in-time
recovery from those that were generated in the original
database history.

To deal with these problems, PostgreSQL has a notion of timelines. Whenever an archive recovery is
completed, a new timeline is created to identify the series of
WAL records generated after that recovery. The timeline ID
number is part of WAL segment file names, and so a new timeline
does not overwrite the WAL data generated by previous
timelines. It is in fact possible to archive many different
timelines. While that might seem like a useless feature, it's
often a lifesaver. Consider the situation where you aren't
quite sure what point-in-time to recover to, and so have to do
several point-in-time recoveries by trial and error until you
find the best place to branch off from the old history. Without
timelines this process would soon generate an unmanageable
mess. With timelines, you can recover to any prior state, including states
in timeline branches that you later abandoned.

Each time a new timeline is created, PostgreSQL creates a "timeline history" file that shows which
timeline it branched off from and when. These history files are
necessary to allow the system to pick the right WAL segment
files when recovering from an archive that contains multiple
timelines. Therefore, they are archived into the WAL archive
area just like WAL segment files. The history files are just
small text files, so it's cheap and appropriate to keep them
around indefinitely (unlike the segment files which are large).
You can, if you like, add comments to a history file to make
your own notes about how and why this particular timeline came
to be. Such comments will be especially valuable when you have
a thicket of different timelines as a result of
experimentation.

The default behavior of recovery is to recover along the
same timeline that was current when the base backup was taken.
If you want to recover into some child timeline (that is, you
want to return to some state that was itself generated after a
recovery attempt), you need to specify the target timeline ID
in recovery.conf. You cannot recover
into timelines that branched off earlier than the base
backup.

At this writing, there are several limitations of the
on-line backup technique. These will probably be fixed in
future releases:

Operations on hash and R-tree indexes are not presently
WAL-logged, so replay will not update these index types.
The recommended workaround is to manually REINDEX each such index after completing a
recovery operation.

If a CREATE DATABASE command is
executed while a base backup is being taken, and then the
template database that the CREATE
DATABASE copied is modified while the base backup is
still in progress, it is possible that recovery will cause
those modifications to be propagated into the created
database as well. This is of course undesirable. To avoid
this risk, it is best not to modify any template databases
while taking a base backup.

CREATE TABLESPACE commands are
WAL-logged with the literal absolute path, and will
therefore be replayed as tablespace creations with the same
absolute path. This might be undesirable if the log is
being replayed on a different machine. It can be dangerous
even if the log is being replayed on the same machine, but
into a new data directory: the replay will still overwrite
the contents of the original tablespace. To avoid potential
gotchas of this sort, the best practice is to take a new
base backup after creating or dropping tablespaces.

It should also be noted that the default WAL format is fairly bulky since it
includes many disk page snapshots. These page snapshots are
designed to support crash recovery, since we may need to fix
partially-written disk pages. Depending on your system hardware
and software, the risk of partial writes may be small enough to
ignore, in which case you can significantly reduce the total
volume of archived logs by turning off page snapshots using the
full_page_writes
parameter. (Read the notes and warnings in Chapter 26 before you do so.) Turning off page
snapshots does not prevent use of the logs for PITR operations.
An area for future development is to compress archived WAL data
by removing unnecessary page copies even when full_page_writes is on. In the meantime,
administrators may wish to reduce the number of page snapshots
included in WAL by increasing the checkpoint interval
parameters as much as feasible.

Comments

Short version(Unix):1. Create your postgres user writeable directory2. Uncomment archive_command and change /mnt/server/archivedir/%f to /your/directory/%f3. Run psql template14. Execute query: SELECT pg_start_backup('something');5. Copy the whole database to a base backup place(do not use the same a the dir in step 1)6. Execute query: SELECT pg_stop_backup();7. Voila!

May 18, 2006, 4:24 p.m.

&gt; Also, some versions of GNU tar consider it an error if a file is &gt; changed while tar is copying it.In some situations (I don't know exactely what causes it), tar even stalls and will not return at all - without raising an error.cpio, on the other hand, even works in those situations.